1. Field of the Invention
This invention relates to a color registration error correction method for correcting an error in color image forming position (hereinafter, referred to as registration) for use with a color image forming apparatus which performs multiple transfers of an image of different colors onto an image support member, such as a conveyed recording member or intermediate transfer substance, for providing a color image.
2. Description of the Related Art
In recent years, documents processed at offices, etc., have quickly become colored and image forming devices such as copiers, printers, and facsimile machines for handling the documents have just as quickly been provided with color functionality. At present, the color machines tend to provide high picture quality and operate at high speed, while at the same time performing high-quality and quick operation of business processing at offices or the like. For color machines meeting these demands, for example, a so-called tandem-type color image forming apparatus is already proposed which has image forming units for each color of black (B), yellow (Y), magenta (M), and cyan (C). This apparatus performs multiple transfers of an image of different colors formed by the image forming units onto a conveyed recording member or intermediate transfer substance for forming a color image.
The tandem-type color image forming apparatus, which forms one image by using a plurality of image forming units, can operate at some degree of high speed. However, if high-speed image forming is intended, alignment of image forming positions of the colors, namely, color, registration often become degraded and the high picture quality cannot be maintained; the high picture quality is extremely incompatible with the high-speed operation. This is because the positions and sizes of the image forming units and further the positions and sizes of the parts in the image forming units subtly change due to a change in a temperature in the machine of the image forming apparatus or an external force applied to the image forming apparatus. The temperature change in the machine and external force are inevitable; for example, an external force to the image forming apparatus is a daily routine such as recovery from a paper jam, parts replacement during maintenance, moving the image forming apparatus, or the like.
For example, Japanese Patent Unexamined Publication No. Sho. 64-88471 has already proposed a registration correction apparatus which reads a shift or misalignment in color image registration and corrects it in order to provide a high quality picture. The registration correction apparatus outputs a predetermined registration misalignment measurement pattern as shown in FIG. 16 or 17 by each of image forming units, detects the registration misalignment measurement pattern with a pattern detecting unit, calculates intervals of the colors on the registration misalignment measurement pattern, and corrects the positions of the image forming units and the image forming timings so that the intervals are equal to a predetermined reference value, thereby providing a high quality picture.
However, the related art has the following problem. Since the intervals of the colors on the predetermined registration misalignment measurement pattern are set to the predetermined reference values, the registration correction apparatus in the related art is prone to be affected by a magnification error in the fast(main) and slow(sub) scanning directions of the pattern detecting unit and a high-precision color registration error correction cannot be made.
The following pattern detecting unit magnification errors are possible:
i) Variation or fluctuation in magnification in the optical system of the pattern detecting unit. It is extremely difficult to hold the magnification in the optical system to a predetermined constant value. PA1 ii) If an array-like sensor, such as a CCD sensor, is adopted for the pattern detecting unit, variation or fluctuation in arrangement pitch of the sensor array may occur. PA1 iii) Magnification error occurring when the move speed of the registration misalignment measurement pattern deviates from the design value. This error occurs only in the slow scanning direction. PA1 i) Long pattern detecting unit is required. PA1 ii) If the pattern detecting unit arrangement is not slow to the move direction of the registration misalignment measurement pattern, the picture elements detected for each color differs, causing an error.
If each of the three sources has a magnification error of 1% and the color registration misalignment is corrected so that the intervals of the colors on the registration misalignment measurement pattern are set to a predetermined reference value, for example, 10 mm, the color registration misalignment does not reach zero; instead it becomes no less than about 200-300 .mu.m. This means that it is difficult to provide a high picture quality by the conventional method even if a correction is applied to the image forming units, etc.
The magnification error in the slow scanning direction can be solved by using a registration misalignment measurement pattern shown in FIG. 18 whereby toner patterns of all colors can be detected at almost the same time, but the method introduces the following new problems: